With the increasing use of mobile communication systems, system operators of mobile communication systems need more transmitters, such as radio stations typically being part of base stations, in order to meet the increased demand for telecommunication. Increasing the number of the base stations is expensive for the system operator. For example, maintenance of the base stations, i.e. the radio stations, may require an engineer to be present at the site of the base station. The increased number of the base stations could therefore require an increased number of service engineers to serve the increased number of the base stations. There are code sharing (Code Division Multiple Access CDMA) and time division (Time Division Multiple Access TDMA) strategies to increase the amount of customers served by an individual one of the base stations.
Nowadays antenna arrays are used in the field of mobile communications systems in order to reduce power transmitted to a handset of a customer and thereby increase the efficiency of the base station, i.e. the radio station. The radio station typically comprises a plurality of antenna elements, i.e. an antenna array adapted for transceiving a payload signal. Typically the radio station comprises a plurality of transmit paths, each of the transmit paths being terminated by one of the antenna elements. The plurality of the antenna elements used in the radio station typically allows steering of a beam transmitted by the active antenna array. The steering of the beam includes but is not limited to at least one of: detection of direction of arrival (DOA), beam forming, down tilting and beam diversity. These techniques of beam steering are well-known in the art.
The code sharing and time division strategies as well as the beam steering rely on the radio station and the active antenna array to transmit and receive within well defined limits set by communication standards. The communications standards typically provide a plurality of channels or frequency bands useable for an uplink communication from the handset to the radio station as well as for a downlink communication from the radio station to the handset. For the radio station to comply with the communication standards it is of interest to reduce so called out of band emissions, i.e. transmission out of a communication frequency band or channel as defined by the communication standards.
For the transmission of the payload signal the base station comprises an amplifier within the transmit paths of the radio station. Typically, each one of the transmit paths comprises an individual one of the amplifiers. The amplifier typically introduces nonlinearities into the transmit paths. The nonlinearities introduced by the amplifier affect a transfer characteristic of the transmit paths. The nonlinearities introduced by the amplifier distort the payload signal relayed by the radio station as a transmit signal along the transmit paths.
The term “transfer characteristics” of a device, such as the amplifier can be construed as follows: Suppose the device has an input port for accepting an input signal and an output port to yield an output signal in response to the input signal. Such a device is referred to as a two-port device if there is only one input port and one output port. The transfer characteristics may likewise be defined for devices comprising a plurality of input ports and/or a plurality of output ports. The transfer characteristics of the device describe how the input signal(s) yield the output signal. It is known in the art that the transfer characteristics of a nonlinear device, for example a diode or the amplifier, generally comprises a nonlinearity.
The concept of predistortion or digital predistortion uses the output signal of the device, for example from the amplifier, for correcting the nonlinear transfer characteristics. Utilizing a feedback process the output signal is compared to the input signal and from this comparison an “inverse distortion” is added and/or multiplied to the input signal in order to linearise the transfer characteristics of the device. By carefully adjusting the predistortion, the nonlinear transfer characteristics of the amplifier can be corrected.
To apply the predistortion to the amplifier it is of interest to know the distortions or nonlinearities introduced by the amplifier. This is commonly achieved by the feedback of the transmit signal to a predistortion module. The predistortion module is adapted to compare the transmitted signal with a signal prior to amplification in order to determine the distortions introduced by the amplifier. The signal prior to amplification is for example the payload signal.
The concept of the predistortion has been explained in terms of correcting the transfer characteristics with respect of the amplitude. It is understood that predistortion may alternatively and/or additionally correct for nonlinearities with respect to a phase of the input signal and the output signal.
The nonlinearities of the transfer characteristics of the transmit path are typically dominated by the nonlinearities in the transfer characteristics of the amplifier. It is often sufficient to correct for the nonlinearities of the amplifier.